Conventional offshore installation methods are normally based on the use of tugger lines directly connecting the structure to winches or handled manually by on-deck personnel. A common challenge for such operations is to obtain adequate control of the movement of the structure and the rotation in particular when the vessel is influenced by wave loads. Especially in situations where the structure is large and there is limited deck space, the overboarding operation is critical due to the risk of clashing of structure into deck equipment. Consequently, the limiting sea state for such operations is often quite low.
A number of constant tension winches, typically five to eight winches, have previously been used to guide and overboard heavy structures. The winches have been arranged such that the structure is guided step by step by a system of winch wires connected between the structure and deck. As the winches at times will pull against each other, wires have ruptured as a result of lack of control due to the complexity of the system.
GB 2502379 A, which was not publically available at the priority date of the present application, shows overboarding a structure by means of a crane while keeping the object stationary with respect to the crane axis by means of telescopic arms extending between the structure and a rotatable ring on the pedestal of the crane. The arms are maintained in compression in order to reduce pendulum movements of the structure caused by ship movements due to the sea state. This system is complicated and cannot easily be decommissioned for use on other vessels. Besides, it cannot be used for unwieldy structures and structures stored outside the relatively short reach of the arms.